Structure-activity studies of normal and retro pig cecropin-melittin hybrids

Chimeric analogs of cecropin P1 and melittin with normal and retro sequences were synthesized to explore the effect of sequence, amide bond direction (helical dipole), charge, amphipathicity and hydrophobicity on their antibacterial activity and channel-forming ability. When viewed from the opposite end by rotation in the plane 180 degrees retro analogs have the same sequence as the parent with reversed amide bond and helical dipole directions. The expected activities were related to the important structural features and a series of assumptions were made. Retro analogs are expected to be inactive if both sequence and amide bond direction make critical contributions to the activity. CP1(1-10)M(2-9) amide, (SWLSKTAKKLIGAVLKVL), showed a broad antibacterial spectrum with high activity against the two Gram-negative and three Gram-positive bacteria tested. Retro-CP1(1-10)M(2-9) was less active compared to its normal peptide. CP1(1-9)M(1-8) and CP1(1-9)M(2-8) amides were found to be active against Gram-negative Escherichia coli and also Gram-positive Streptococcus pyogenes, but inactive against the other test organisms. The corresponding retro analogs were inactive against all the five bacteria tested. These results suggest that both sequence and amide bond direction (helix dipole) are important structural requirements for the activity of CP1-M hybrids. Acetylation of the N-terminal amine in both normal and retro analogs lowered their activity, indicating the contribution of free amine to the activity. These analogs form ion-conducting channels in lipid bilayers. The action of the peptides may be explained by self-aggregation and formation of ion-conducting pores across bacterial membranes. Conformational analysis obtained from CD measurements showed that all analogs form amphipathic alpha-helices in presence of 12-20% hexafluoro isopropanol. The retro CP1(1-10)M(2-9) amide showed higher helicity and is more potent compared to other retro analogs synthesized. These studies show the effect of small sequence modifications on the biological activity of the peptides and on their alpha-helical conformation in HFIP, the structure-inducing organic solvent.